1. Field of the Invention
The present invention relates to a method and related communication device in a wireless communication system for improving uplink transmission, and more particularly, to a method and related communication device for improving uplink transmission when semi-persistent scheduling resources are configured.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, called evolved Node-Bs (eNBs), and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The radio protocol stacks of the E-UTRAN is given including a radio resource control layer (RRC), a packet data convergence protocol layer (PDCP), a radio link control layer (RLC), a medium access control layer (MAC), and a physical layer (PHY). In the 3GPP associated specifications, logical channels are defined as service access points between the MAC layer and the RLC layer. The MAC layer provides data transfer services on logical channels. Each logical channel type is defined by the type of information to be transferred. The RRC layer controls the scheduling of uplink (UL) data by giving each logical channel a priority.
A dynamic scheduling function is used in MAC for utilizing shared channel (SCH) resources. When sharing resources between UEs, MAC in eNB dynamically allocates physical layer resources for the downlink (DL)-SCH and UL-SCH depending on the traffic volume, the Quality of Service (QoS) requirements of each UE, and associated radio bearers. In order to utilize SCH resources more efficiently, a semi-persistent scheduling (SPS) function is introduced in the LTE system and is used in MAC, for serving upper layer applications which generates semi-static size data periodically, e.g. VoIP services.
A buffer status reporting procedure is used to provide a serving eNB with information about the amount of data in UL transmission buffers of a UE. There are three types of buffer status reports (BSRs) for different triggering events in current specification: a regular BSR, a periodic BSR, and a padding BSR, which are described later. A MAC protocol data unit consists of a MAC header, zero or more MAC service data units, zero or more MAC control elements (CEs), and optionally padding bits. BSR is one of MAC CEs, and other MAC CEs are a cell radio network temporary identifier (C-RNTI) and a power headroom report (PHR), etc. A scheduling request (SR) is used for requesting UL resources and is triggered by the regular BSR when the UE does not have an UL resource which is allocated for a current transmission time interval (TTI).
The regular BSR is triggered when UL data belonging to a logical channel with higher priority than those for which data already existed in a UE transmission buffer arrives at the UE transmission buffer. The padding BSR is triggered when UL resources are allocated and a number of padding bits is larger than a size of BSR MAC CE. The periodic BSR is optional in the LTE system, which is used to provide the eNB with updated status of the UE transmission buffer periodically. A periodic BSR timer is started when the regular BSR is triggered. When the periodic BSR timer expires, the periodic BSR is triggered and waits for an allocated UL grant to be transmitted. When the periodic BSR is transmitted, the periodic BSR timer is restarted. Once the regular BSR is triggered, the periodic BSR shall be generated periodically. Note that, the periodic BSR may be cancelled by certain events, e.g., a triggered regular BSR may cancel other existed BSRs.
Current specification defines a retransmission BSR timer “RETX BSR TIMER” for handling the situation that the UE has a transmitted BSR but the eNB does not respond with any corresponding UL resource for a long time. The retransmission BSR timer is usually much longer than a period of SPS resources. The retransmission BSR timer is started or restarted when the regular BSR is triggered or when a new UL grant is allocated. When the retransmission BSR timer expires, an enforced BSR-SR event is triggered for requesting UL resources, so that data in the transmission buffer has an opportunity to be transmitted.
Please refer to FIG. 1, which is a timing diagram illustrating uplink transmission of lower priority data according to the prior art. According to current specification, in the condition that SPS resources are configured and SPS data available for transmission in the transmission buffer is never emptied, when lower priority data belonging to a logical channel except SPS logical channels arrives at the transmission buffer, no regular BSR is triggered. As a result, the eNB does not receive information of lower priority data such that lower priority data may have no opportunity to be transmitted and simply get stalled in the buffer. Due to logical channel prioritization, stall of lower priority data happens, whether SPS resources are configured. SPS resources configuration does deteriorate this problem, however.
Note that, the original idea of the periodic BSR is to periodically report status of the UE transmission buffer to the eNB. When the periodic BSR is cancelled or is not configured, the eNB losses opportunity to periodically receive updated status of the UE transmission buffer and therefore, under the continuous presence of higher priority data as SPS data, the eNB does not find out the existence of lower priority data, and lower priority data is stalled in the transmission buffer. Besides, the retransmission BSR timer “RETX BSR TIMER” is unhelpful for requesting UL resources when SPS resources are configured because the retransmission BSR timer may be restarted periodically by arriving SPS resources and cannot expire, which results in the situation that no BSR-SR is triggered and lower priority data may be stalled forever.
Current MAC specification has a statement: “MAC control elements for BSR, with exception of padding BSR, have higher priority than U-plane logical channels.” That is, the MAC CE, except the padding BSR, is of higher priority than any data radio bearer. Therefore, when generating a transport block, the UE MAC would first insert the BSR and then considers data from logical channels.
Please refer to FIG. 2, which is a timing diagram illustrating uplink transmission of SPS data and a MAC CE, e.g., a BSR, according to the prior art. FIG. 2 illustrates that for a current TTI, there are SPS data, which is assumed to have higher priority, and data from other logical channel except SPS logical channel existed in the transmission buffer; an SPS resource is allocated, which has a size equal to a size of SPS data in the transmission buffer; and a BSR is triggered. In this situation, the UE MAC inserts the BSR with a long BSR format and fills the rest of the SPS resource with SPS data. When another UL grant is allocated later, as shown in FIG. 2, the UE MAC fills the UL grant with the rest of SPS data and fills the data from other logical channel. From the above, the transmission of one SPS packet is split into two attempts, which increases the probability of retransmission and incurs extra delay.
An information element (IE) “MAC-MainConfiguration” is used to specify the transport channel configuration for data radio bearers, including periods of SPS resources and periods of periodic MAC CEs, such as the periodic BSR and a periodic PHR. In the IE “MAC-MainConfiguration”, a period of a periodic MAC CE is set by a periodic MAC CE timer, and some periods of the periodic MAC CEs are not larger than the period of SPS resources. For example, when a periodic MAC CE timer is 5 second, which is not larger than 10 second as the period of SPS resources, the periodic MAC CE timer always expires before an allocated SPS UL resource. Furthermore, when the allocated SPS resource is not large enough for transmitting SPS data plus the periodic MAC CE, once the periodic MAC CE is inserted at the SPS resource, subsequent insertion at every SPS resource happens inevitably. Please refer to FIG. 3, which is a timing diagram illustrating uplink transmission of SPS data and periodic BSRs according to the prior art. The split behavior may happen periodically until the eNB modifies SPS resources, or even in the rest span of the configured SPS resources.